The general aim of the proposed research is to increase our understanding of how gonocytes interact with other testicular cells, especially Sertoli cells, during the early perinatal period when these germ cells re-enter the cell cycle and move into the presumptive basal compartment where they contact the basement membrane (BM) underlying the seminiferous epithelium. These events are critical for development of a fertile testis in the adult since they lay the foundation for spermatogenesis and hence insure, at least in part, adequate production of sperm. Specifically, the following aims will be pursued: 1) the relationship between gonocyte development, i.e., initiation of division and movement to the BM, and differentiation of nearby Sertoli cells will be probed by determining the spatial organization of entry into the cell cycle by gonocytes and exit from the cycle, or terminal differentiation, of Sertoli cells; 2) the hypothesis that gonocytes reach the BM by active migration will be tested, the cellular mechanisms involved identified and the source of presumed stimulatory factors located; 3) Sertoli cell-gonocyte interaction will be studied by identifying the nature of surface moieties mediating their adhesion and testing the possibility that they communicate via gap junctions; 4) the nature of gonocyte-matrix adhesion in vitro will also be probed, and this interaction related to that occurring in vivo. A number of different approaches will be utilized for these studies, including autoradiography and immunocytochemistry to locate proliferative and quiescent cells in vivo, and assays will be developed to study presumed migratory behavior in vitro, a well as cell-cell and cell-substrate adhesion. Both enzyme- and immune-cytochemistry will be used to study polarization of gonocyte organelles during elaboration of pseudopod-like processes in vivo and in vitro, during presumed migration to the BM or substrate. Where possible, studies will be carried out in vivo, although gonocytes isolated in short-term culture or cocultured with Sertoli cells will also be used. Data emerging from these studies will shed new light on events that occur during early testicular development which are crucial for establishment of fertility, and will also provide insights into how these events are regulated.